Exterior watch parts such as wristwatch case frames, back lids and bands, ornaments such as necklaces, bracelets, pierced earrings, finger rings and earrings and spectacle frames are decorative members which are brought into direct contact with the skin when worn. Most of these decorative members have a tone such as white gloss, golden gloss or black gloss.
In the industrial manufacturing of these decorative members, the common process is employed in which a coating layer with the above tones are formed on a substrate, for example, a substrate of a metal in accordance with the wet or dry plating technique.
For example, in the manufacturing of a watch case with white gloss, the common process comprises the formation as a primary layer of a Ni plate layer having a thickness of about 2 to 3 .mu.m on a surface of a substrate of brass or German silver and thereafter the formation as a finishing layer of a Pd--Ni alloy plate layer having a thickness of about 2 to 3 .mu.m on the Ni plate layer, optionally followed by flash plating of Rh or Pt on the above Pd--Ni alloy plate layer.
In the manufacturing of a decorative member with golden gloss, the common process comprises the formation of a Ni plate layer having a thickness of about 2 to 3 .mu.m on a surface of a substrate of, for example, brass or German silver, thereafter the formation of a Pd--Ni alloy plate layer having a thickness of about 2 to 3 .mu.m on the Ni plate layer and finally the formation of a layer of, for example, 22 carat Au--Ni alloy plate or an Au plate layer having a thickness of about 1 to 2 .mu.m on the Pd--Ni alloy plate layer. Further, a golden decorative member has been proposed which is produced by sequentially forming an Ni plate layer and then a Pd--Ni alloy plate layer on a surface of a substrate according to the wet plating technique and thereafter sequentially forming an about 0.1 to 10 .mu.m thick TiN layer and Au or Au--Ni alloy layer in this order on the Pd--Ni alloy plate layer according to the dry plating technique such as sputtering or vacuum evaporation technique.
In the manufacturing of a decorative member with black gloss, the common process comprises the formation of about 2 to 3 .mu.m thick Ni plate layer or Cu--Ni--Cr alloy plate layer on a surface of a substrate of, for example, brass or German silver and forming thereon an about 0.5 to 3 .mu.m black Ni plate layer or black Cr plate layer. Further, a black decorative member has been proposed which is produced by forming the above Ni plate layer or Cu--Ni--Cr alloy plate layer on a surface of a substrate according to the wet plating technique and forming thereon a TaC layer having a thickness of about 0.1 to 2 .mu.m according to the PVD technique such as ion plating or sputtering technique.
However, all the above conventional white, golden and black decorative members contain Ni, so that they have the danger of inducing dermatitis because the Ni may be dissolved as free Ni in, for example, the sweat excreted from the skin and brought into contact with the skin.
White, golden and black decorative members which do not contain Ni have recently been proposed for suppressing the above Ni allergization.
For example, white decorative members are known which are produced by forming a Cu plate layer or an Sn--Cu alloy plate layer containing 30 to 35% by weight of Sn in a thickness of at least 2 .mu.m on a surface of a substrate of, for example, brass and forming thereon a finishing layer of a Pd plate layer having a thickness of about 0.1 to 5 .mu.m. Also, white decorative members are known which are produced by forming a flash plate layer of Rh or Pt on the above Pd plate layer.
Golden decorative members are known which are produced by first forming a Cu plate layer or a plate layer of Cu alloy such as the above Sn--Cu alloy on a surface of a substrate of, for example, brass, and subsequently forming a Pd plate layer thereon and thereafter forming an Au plate layer or a plate layer of Au alloy not containing Ni, such as Au--Fe alloy, on the Pd plate layer.
Also, golden decorative members are known which are produced by sequentially forming an about 0.1 to 10 .mu.m thick TiN layer and a layer of Au per se or Au alloy not containing Ni in this order on the above Pd plate layer according to the dry plating technique.
Black decorative members not containing any Ni component are known which are produced by forming the above TaC layer directly on the above Pd plate layer according to the dry plating technique.
All the above decorative members do not contain any Ni component, so that, when worn so as to be brought into contact with the skin, they do not induce Ni allergy.
However, these decorative members, without exception, form a plate layer from Pd which is an expensive noble metal, so that the product cost thereof is inevitably very high.
Thus, efforts are being made to use a plate layer of Pd alloy in place of the plate layer of Pd per se for reducing the relative content of Pd in the plate layer to thereby lower the whole product cost thereof.
A known example of the above plate layer of Pd alloy is a plate layer of Sn--Cu--Pd alloy. However, this plate layer of Sn--Cu--Pd alloy is unsatisfactory in the reduction of product cost because the Pd content thereof is as conspicuously high as about 50 to 99% by weight.
In the above situation, the inventors made investigations on Sn--Cu--Pd alloy plate layers having a low Pd content and developed a method of plating an Sn--Cu--Pd ternary alloy composed of 10 to 20% by weight of Sn, 10 to 80% by weight of Cu and 10 to 50% by weight of Pd and a plating bath for use in this method. These are described in Japanese Patent Application No. 5(1993)-80844.
The Sn--Cu--Pd alloy plate layer proposed therein has a low density as compared with that of a plate layer of pure Pd and, for example, when compared in plate layers of the same thicknesses, the Sn--Cu--Pd alloy plate layer has its Pd content reduced by about 60%, so that the product cost can be lowered. The Sn--Cu--Pd alloy plate layer has excellent corrosion resistance and is furnished with excellent specular gloss.
The object of the present invention is to provide a decorative member which may have white, gold, black or a multicolor combination of the above colors and which does not induce Ni allergy and can be produced at lowered cost, by utilizing the above excellent properties of the Sn--Cu--Pd alloy plate layer.